During the 2½ years of clashes in Syria, there has been constant debate about how Turkey's borders were crossed.

There were reports that Islamic groups going to fight regime of President Bashar al-Assad -- first and foremost al-Qaeda, which has supporters in Turkey -- were crossing over the Turkish border.

Interesting claims

To find out more, we met with people close to al-Qaeda in Istanbul. These people are shopkeepers who live in the Fatih district of Istanbul, but who won't give their names.

They have interesting things to say about the Syrian war. These sources told us that following the eruption of war in Syria, al-Qaeda elements from Europe, the Caucasus, Afghanistan and North Africa began crossing into Syria via Turkey.

These sources also had interesting things to say about the clashes with the Kurdish PYD and how the border is crossed.

Met by intelligence officials

O.E., one of our sources, said he crossed the border and went to Syria before the Jabhat al-Nusra-PYD clashes.

He crossed from an unsupervised area on the Turkish side to the Syrian side controlled by the PYD. O.E. said, "We told the PYD we were there for Jabhat al-Nusra and they let us pass."

O.E. said many people cross the same way: "Fighters coming via Chechnya and Afghanistan are met at the Syrian border. There are intelligence officials there.

Those crossing the border inform the intelligence people of their affiliation and under whose command they will be. Then, they cross the border and report to their units."

Treated in Turkey

O.E. said those heavily wounded in clashes are brought to Turkish hospitals. He added, "Some return to their countries by the same route.

There are al-Qaeda mujahedeen from Afghanistan and the Caucasus fronts who come with their families. Most of them settle in Syria.

There are hundreds of militants who come the same way from Northern Africa, the Caucasus, Europe and Afghanistan. They simply cross the Turkish border and join the fight."

1,000 Chechens to Syria

O.E. said Chechens are now one of the strongest groups in Syria. "Under their commander Abu Omar, about 1,000 Chechens came to Syria.

First they were with Jabhat al-Nusra, but now they have moved over to the Islamic State of Iraq and Syria (ISIS)," he said.

There are also Turks

O.E. said many Turks had gone to Syria to fight. "Some were martyred. Some stayed for a while and returned. Some couldn't resist going back to Syria.

A retired policeman who is a friend of mine went to Syria to fight. He trained fighters in weapons. Several of us went to Syria before the fighting between the PYD and Jabhat al-Nusra broke out.

Without being asked anything on the Turkish side, we just crossed to an area of Syria controlled by the PYD. We told them we came to [fight with] Jabhat al-Nusra and they let us enter," O.E. said.

The ISIS fans the clashes

O.E. claimed that it was the ISIS that was flaming the clashes with the PYD. "The ISIS declared that Jabhat al-Nusra was its subordinate organization.

Jabhat al-Nusra commanders refused this claim and said they were under al-Qaeda leader Ayman al-Zawahri.

These claims caused disputes within the organization. Chechen groups under Abu Omar in Syria split from Jabhat al-Nusra and joined the ranks of the ISIS.

It was the ISIS fighters who provoked the recent clashes with the PYD.

Reports said the ISIS entered and opened fire in PYD-controlled villages to disrupt the non-hostility agreement between the PYD and Jabhat al-Nusra," he concluded.